This disclosure relates to equipment and procedures for achieving an improved solder mask coating on printed wiring boards (PWB). More specifically, this disclosure teaches how paste-consistency photopolymer coatings are applied over the entire board surface; how the photopolymer coating is selectively exposed to UV light to harden wanted photopolymer, and how unhardened photopolymer is removed.
In order to place this disclosure in proper perspective, a brief description of the current art of solder mask application and needed improvements will be presented.
In the manufacture of printed wiring boards, it is becoming increasingly commonplace to solder mask the finished boards exterior surfaces. Typically, the solder mask covers the base laminate and conductor traces, leaving the circuit pads free. After electrical components are added to the PWB, the assembly is conveyed over a wave of molten solder which solders all components in seconds. The solder mask sheds the molten solder, so that only circuit pads receive solder. The solder mask thus prevents solder bridges between adjacent conductors and reduces the amount of rework and post-soldering inspection required.
The solder mask remains in place after soldering and serves as a life long insulator and environmental barrier.
Traditionally, solder masks have consisted of epoxy thermosets applied to the PWB by screen printing and cured by oven baking. More recently, UV-curable photopolymers have been introduced as a way to overcome the lengthy bake cycle required by the epoxies. These photopolymers are generally screen printed and cured in 10 seconds when exposed to a 200 watt per inch UV lamp source.
The full potential of the UV-curable photopolymers cannot be achieved by screen printing because of requirements imposed by PWB users. First, it is generally necessary to apply a solder mask coating of approximately 0.002 inches thick to achieve a continuous coating free of "Skips", which are areas where the mask fails to fill the spaces between conductors down to the base laminate.
The surface of a typical PWB is highly irregular, having conductor heights of 0.003 to 0.005 inches above the base laminate. This surface irregularity decreases printing resolution and causes "smearing" of the epoxy and photopolymer onto circuit pads thereby preventing good soldering and partially defeating the purpose of solder masking. Thus, if skip-free printing is achieved, then smearing is frequently experienced.
Photoimaging of the photopolymer is the best way to achieve impressions free of skips and smears, while maintaining high accuracy. Three techniques currently being used will be described.
In the present art UV-curable photopolymers are being photoimaged to provide printed wiring board cover coats and solder masks, avoiding both skips and smears. Known techniques include covering the PWB with photopolymer via roller coating, then laminating a thin plastic sheet over the coated board, exposing thru a photographic film in contact with this thin plastic sheet.
Several disadvantages result from this technique:
1. The thin plastic sheet causes a loss of resolution, since the photographic film emulsion is separated from the photopolymer by the film thickness, and light inadvertently hardens a portion of unwanted photopolymer.
2. The thin film is scrapped, wasting one square foot of film for each square foot of board surface coated.
3. Entrapped air results unless the protective film is vacuum laminated.
4. During the exposure cycle, the pressure of the photographic film against the photopolymer forces the photopolymer into the PWB holes, wasting photopolymer and increasing the washout time required. The photopolymer is also thinned out over top of the conductors by this extrusion process.
Another method is used for photoimaging UV-curable photopolymers, in which a collimated light source is used to expose the photopolymer through a photographic film positioned above the printed wiring board; a process known as off-contact exposure. There are two primary disadvantages to this approach:
1. The collimated light source is expensive, and generally priced beyond the reach of smaller PWB manufacturers.
2. A loss of resolution is experienced because of the off-contact separation and photographic film and photopolymer, even with a collimated light source.
In the present art there is one other alternative to screen printing solder mask which solves both the problems of smearing and skipping; "dry film soldermask" as manufactured by the DuPont Company. Dry film soldermask consists of a tacky photopolymer film sandwiched between two carrier films, and supplied in roll form. The dry film is laminated onto the board via a vacuum laminator; selectively exposed to UV light and developed for 2.5 minutes in a solvent spray bath. Smears are largely eliminated, as are skips, and the resolution is far better than screen printing.
However, there are several serious problems with this dry film:
1. The dry film costs approximately 10 times that of epoxy and 5 times that of UV-curable photopolymer.
2. Labor required to laminate, expose and develop the dry film is excessive.
3. The processing equipment is excessively expensive.
4. For every square foot of photopolymer used, there are two square feet of accompanying plastic film which are wasted.
5. For every square foot of board surface covered approximately 1.25 square feet of dry film is required. Losses are attributed to the vacuum laminator and the need for the roll width to exceed the board width.
6. When simultaneously exposing the dry film on two sides of the board ultraviolet light travels through the base laminate and partially exposes the dry film on the opposite side unless an opaque laminate is used. This thru- laminate exposure causes partial polymerization and results in smears and loss of resolution.
In the present art, glass photographic plates are used to expose photopolymers, in both the contact and off-contact modes. The current art glass photographic plate has the opaque emulsion affixed to the glass, essentially Coplaner with the lower glass surface, since the emulsion is of the order of several microns thick. The use of a conventional glass photographic plate has several disadvantages:
1. If used in off-contact mode, a collimated light source is required for good resolution.
2. If used on contact:
2A. The glass must be lowered to contact the photopolymer surface in a vacuum chamber or air will be trapped, and the gloss surface marred. PA1 2B. Excessive solder mask photopolymer is required, since the glass lower surface must "float" on photopolymer without touching the underlying conductors. If conductor traces are 0.004 inches high, then the photopolymer thickness must be 0.005 inches to ensure that conductors receive at least 0.001 inches of photopolymer. PA1 2C. The photopolymer is forced into circuit holes, making removal more difficult.
Accordingly, one objective of this invention is to provide an exposure plate which can be used to produce a photopolymer coating which is free of skips, smears, pinholes and entrapped air, while retaining a glossy surface which is esthetically pleasing.
Another objective is to provide an increased photopolymer thickness due to extrusion of photopolymer from the opaqued areas by action of the exposure plate image. This increased thickness enhances the solder mask by providing a thicker mask between circuit pads and adjacent conductors.
Another objective is to produce a photopolymer coating having high resolution with a non-collimated light source, using both conveyorized and drawer-type lamps.
Another objective is to obtain a high production rate with unskilled labor.
Another objective is to be able to expose the photopolymer with a conveyorized UV curing lamp system so that the exposure cycle is reduced to the order of 10 seconds, using conveyorized UV curing lamps currently in use in PWB manufacturing.
A corollary objective is to provide an exposure plate and exposure assembly capable of withstanding the high temperatures encountered in repeatedly conveying the exposure assembly under high intensity lamps.
Another objective is to minimize waste of photopolymer and to eliminate the wasteful practice of consuming plastic film.
Another objective is to provide equipment which will work well with many photopolymers available commerically, as well as new UV curing materials which may become available in the future.
A further objective is to provide a photopolymer coating which is cured in two stages. The first stage is cured to the extent that unexposed photopolymer can be washed out without affecting the exposed photopolymer. After washout and inspection, the final cure is accomplished.
A further objective is to provide a photographic step tablet which produces an indication that each PWB is properly exposed.
Another objective is to provide improved methods for photoimaging UV-curable plating resists and etch resists used in the manufacture of printed wiring boards.
Those patents representative of the prior art photoprinting techniques in the printed circuit board art are:
U.S. Pat. No. 3,965,277--Guditz et al., June 22, 1976 which processes printed circuits by UV photo resist exposure; and
U.S. Pat. No. 3,936,301--Schneider, Feb. 3, 1976 which employs contact type photolithographic masking processes to develop mask patterns.